1. Field of the Invention
This invention relates generally to the fabrication of a giant magnetoresistive (GMR) magnetic field sensor of the spin-filter synthetic bottom spin-valve (SFBSV) type and more specifically to a method for forming a TaO specularly reflecting layer in direct contact with a free magnetic layer by an in-situ process of natural oxidation.
2. Description of the Related Art
Magnetic read heads whose sensors make use of the giant magnetoresistive effect (GMR) in the bottom spin-valve configuration (BSV) are being increasingly required to read information recorded on magnetic media at ultra-high area densities (e.g. >45 Gb/in2). The typical BSV sensor configuration includes (in vertically ascending order) a pinning layer, a pinned layer, a conductive spacer layer, a ferromagnetic free layer and a capping layer. Sensing current is introduced into and extracted from this configuration by laterally disposed leads. Again, typically, the pinning layer is a layer of antiferromagnetic material (AFM) which pins (fixes in space) the magnetic moment of the pinned layer (typically a layer of ferromagnetic material) in a direction normal to the plane of the air-bearing surface (ABS) of the sensor. The magnetic moment of the ferromagnetic free layer, not being pinned, is free to rotate with respect to that of the pinned layer under the influence of external magnetic fields and it is those rotations that cause the resistance of the sensor, R, to vary (dR) and, in combination with the sensing current, to produce an electrical signal. The GMR effect, which is relied upon to give maximum resistance variations, dR, for given rotations of the free layer magnetic moment, is a result of the scattering of conduction electrons in the spacer layer by the surfaces of the pinned and free layers that bound it. This scattering is spin-dependent and a function of the relative orientations of the two magnetic moments.
In order for the dR to be reproducible and invariant under symmetric changes in the external field, the magnetic moment of the free layer should return to the same position (the bias point) when no external magnetic signals are present (the quiescent state). The bias point of the free layer is typically made to be perpendicular to the pinned moment of the pinned layer, ie. in the plane of the ABS.
To be capable of reading ultra-high area densities, the BSV sensor must be able to resolve extremely high linear bit densities, bits-per-inch, (BPI) and track densities, tracks-per-inch, (TPI), which, in turn, requires that it have an extremely narrow trackwidth and ultra-thin free layer (thickness <20 angstroms) to maintain high signal output. Unfortunately, as the free layer is made increasingly thin, it becomes difficult to obtain a controllable bias point, a high GMR ratio (dR/R) and good softness (low coercivity). Utilizing synthetic antiferromagnetic (SyAF) pinned layers (ferromagnetic layers coupled with their magnetic moments antiparallel) can reduce magnetostatic fields between the pinned and free layers which adversely affect the biasing; but if the free layer is sufficiently thin, even the magnetic fields produced by the sensing current have an adverse affect.
To overcome this significant problem, a spin-filter spin valve (SFSV) configuration has been introduced in which the free layer is placed between the usual Cu spacer layer and an additional high-conductance-layer (HCL). This configuration reduces the sense current field in the free layer by shifting the sense current center towards the free layer. This results in the sense current producing a small bias point shift. In addition, the SFSV configuration allows the use of an ultra-thin CoFe free layer which, when combined with a properly formed HCL, has an advantageous small positive magnetostriction combined with a high output. The spin-filter spin valve in the bottom spin valve configuration utilizing a synthetic antiferromagnetic pinned layer will hereinafter be denoted a “synthetic SFBSV” structure.
Attempts have been made in the prior art to improve the GMR ratio of the synthetic SFBSV structure. One approach has been the formation of a specularly reflecting capping layer between the free layer and the conducting lead layer. Such a layer acts like a mirror and effectively lengthens the paths of conduction electrons within the free layer so that the resistive variations of GMR effect can yield a maximal output signal. As is disclosed in the related Patent Applications, Ser. No. 10/460,086 and Ser. No. 10/124,004, filing date Apr. 17, 2002, both fully incorporated herein by reference, a particularly advantageous method of forming such a specularly reflecting capping layer is by oxidizing a Ta capping layer during an Ar/O2 reactive ion etch (RIE) process used to pattern a lead overlay (LOL) layer on the sensor. Such an RIE process is particularly advantageous is forming patterned lead layers that define the ultra-narrow (between 0.19 and 0.11 microns) magnetic read widths required to read densities up to and beyond 100 Gb/in2. The etch thereby serves the dual purpose of patterning the lead layer and, in the process, oxidizing the capping layer beneath it. The advantageous effects of such TaO specularly rejecting layers are also taught by Horng et al. (U.S. Pat. No. 6,466,418) who incorporate such a layer within a bottom spin valve structure which is not of the spin-filter spin valve configuration. Kamiguchi et al. (U.S. Pat. No. 6,348,274) also teach the specularly reflecting properties of oxide layers formed on a variety of magnetic layers withing GMR structures. They particularly teach the advantages of such specularly reflecting layers within the pinned layer of a GMR spin valve structure. Pinarbasi (U.S. Pat. No. 6,268,985) teaches a laminated capping layer which does not increase the coercivity of the free layer upon annealing. Fukuzawa et al. (U.S. Pat. No. 6,338,899) discuss the advantageous nature of oxidized metallic layers and also teach the formation of TaO layers in a variety of spin valve configurations.
The particular method by which a Ta capping layer is oxidized can have an important effect on the performance of the sensor. The present inventors have found that oxidation by plasma etching produces an oxide layer that is of low density and non-uniformity. If, for example, the Ta layer is under-etched, producing an under-oxidized surface, then specular reflection is not as effective as that produced by naturally oxidized Ta. Conversely, if the Ta layer is over-etched (over-oxidation), the energetic oxygen ions in the RIE plasma may damage the free layer beneath the capping layer. In either under or over oxidation, the S/N (signal to noise) ratio of the sensor is degraded. The present invention will disclose a method for naturally oxidizing a Ta layer in-situ, to produce a specularly reflecting layer capped with a uniform oxide formation of a thickness that yields a greatly enhanced GMR ratio (dR/R).